Before reading any further, you are advised to watch the short video above, which shows a seminal experiment that Daniel Simons and his co-author and fellow psychologist Christopher Chabris conducted more than a decade ago (further examples can be found here). It is an experiment that any viewer can take part in – with often startling results. It opened up to the two American professors the phenomenon of "inattentional blindness", alerting them to the many tricks that our brains routinely play on all of us.

If you are reading on, Simons reveals more about the gorilla.

Tell me more about your experiment.

The study that inspired the name of our book was one that Chris Chabris and I did about a dozen years ago, and it was based on some much earlier work by our colleague Ulric Neisser from the 70s. It was a fairly simple study, really.

We had people watch a video in which three people were wearing white shirts and passing a basketball and the only task was to count how many times the players passed the ball. We made it a little harder by having the players fake some passes and move around each other and bounce the ball and dribble it. We also had three people wearing black shirts passing their own ball, but you were only supposed to count how many times the players wearing white passed the ball.

About halfway through the video we had a person wearing a gorilla suit walk into the scene, walk to the middle of the scene, turn to face the camera, thump its chest and then walk off, a total of about nine seconds later.

What we found was that when the people were busy counting the number of passes by the players wearing white, about half the people didn't see the gorilla at all; when we showed it to them later, they were shocked. They accused us of rewinding the tape or switching the video that we showed them. It's surprising to people because we have the strong intuition that if something unexpected and distinctive happens right in front of us, we will automatically notice it. People who saw the gorilla couldn't believe that anyone could miss it and the people who missed it were shocked that they had missed it, as it was so counterintuitive to them.

What does this say about how our brains process the world around us?

Well, several things. One is that we don't actually see as much of the world as we think we see. We're very good at focusing our attention on a limited aspect of the world: we focus our attention on a few things that we want to see and the result of that is that we have to filter out things that we don't care about. And we sometimes also filter out things that we might care about.

This is known as inattentional blindness. The idea is that if you're not focusing attention on something, you often don't consciously perceive it; what's interesting is that our intuitions about this sort of failure of awareness are often wrong.

We've known about these sorts of failures of awareness for years, but if you make it such a vivid thing that people are astonished that they could miss it, it reveals what we intuitively believe we see as opposed to what we actually see. We think we are going to notice anything important that happens in front of us but the reality is that we don't much of the time. Things actually don't grab our attention.

From an evolutionary point of view, does this make sense – to not notice a predator right in front of you, for instance?

If you think about what sort of resources it would take to notice everything in our environment, while at the same time being able to focus intently on the things that we care about, it would take a tremendous amount of resources. We need to be able to focus on what matters and not necessarily on things that are distracting. We don't care if there are leaves blowing in the distance, if we're focusing on an animal on the scene.

So there's some reason to think it makes sense to build a system that uses focused attention this way. But if you also think about it, evolutionary, it was a much simpler time. You're moving at pedestrian speeds... and if you don't spot something for a few seconds, it's not going to matter most of the time. Whereas if you're driving at 100km an hour and someone walks out into the street, it matters if you don't notice, even for a fraction of a second.

Can you give other examples of this?

Everybody, at some point in their life, has spotted an error in a movie: somebody's wearing a jacket, and then they're not... or somebody is eating a pastry and then they're eating a pancake. These errors creep into movies because they're filmed over many days, out of sequence, and cut together later in the editing room....

I have had people saying that they are great at spotting these editing mistakes, but I can sit them down in front of a one-minute movie I made with a colleague that has nine intentional errors in it and they don't spot any.

Why do we have this intuition that we spot all of them? Well, it's a lot like the gorilla case. We're only aware of those things we've noticed, and we're not aware of all the ones we've missed. So if you spot a continuity error, you'll remember that you noticed it, but you won't remember all the ones you never saw. But people tend to generalise from the cases they're aware of and assume that they notice all of them. It's detecting a pattern... but only getting half the evidence.

The Invisible Gorilla is published by HarperCollins.To hear more of this interview, listen to the Guardian's Science Weekly podcast